Hekla produces occasional violent eruptions in Iceland.
After tephra falls during the 1947 eruption of Hekla, fluorine contents of stream waters in areas that received between 1 to 10 cm of ash were found to be between 1.0 and 4.5 ppm for a short duration (Thorarinsson, 1979), elevated from the normal background level of 0.2 ppm. In the 1970 eruption of Hekla, excess fluorine levels (up to 8 ppm) were recorded in the Galtalaekur River. Levels returned to normal with a few hours (Oskarsson, 1980). Although fluorine contamination of water supplies was short lived and caused only a minor environmental disturbances, fluoride contamination of pastures did cause major stock losses (Blong, 1984; Thorarinsson, 1979).